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Guru
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Where Do the Electrons Go?

02/12/2007 6:41 AM

Okay, here's an interesting problem I was set many years ago and, to be honest, I've forgotten the solution so its open to anyone to answer it

In a Cathode ray tube, be it a monitor, TV or oscilloscope tube, there is an electron gun that emits electrons which are accelerated down the tube focused and deflected as necessary, to the front glass coated in a phospher to emit a light when bombarded by the electrons...

The focussed beam of electrons is of course a current flow, but where do the electrons go?

Do they build up at the point on the glass screen and if so why doesn't a static charge build up at that point deflecting any further electron bombardment?

OR

Are they converted to photons? maybe they leak away through the glass to atmosphere? Could they just stay put and sit on the face of the tube until the tube is scrapped?

We all know that touching the glass front of a TV tube gives a static 'tingle' as well as a crackle, could the electrons form a capacitor with the glass front of the tube as a dielectric?

John.

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#1

Re: Where do the Electrons Go.....??

02/12/2007 7:43 AM

In a typical vacuum tube (which is what a cathode ray tube is), electrons are emitted by the electrode and a plate where the electrons are collected.

A cathode ray tube has the same arrangement. The front plate is conductive and is connected to the high positive voltage connector on the body of the tube.

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#2
In reply to #1

Re: Where do the Electrons Go.....??

02/12/2007 8:51 AM

On a CRT the pda (post deflection anode) does not connect to the front face of the tube...

Otherwise that would mean that the front face would have to be metalised, which it isn't...

I stand to be corrected though... John.

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#5
In reply to #2

Re: Where do the Electrons Go.....??

02/12/2007 7:52 PM

The front face is metalized! I can't remember what kind though. I once visited a CRT manufacturer (trying to sell them an infrared temperature sensor) and one of the operators mentioned that. I couldn't see how an electron can get past the plate and on to the phosphor layer but that's what he said.

I think I'll check with Wikipedia. I'm obviously not an expert but I'm only saying what I know.

I'll be back to let you know. 'Gotta go.

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#9
In reply to #5

Re: Where do the Electrons Go.....??

02/13/2007 12:45 AM

Yup, this link says it's metalized though doesn't say what kind. There's also a mention of aluminum.

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#6
In reply to #2

Re: Where do the Electrons Go.....??

02/12/2007 11:10 PM

The front face is metallized because, if it weren't, then you'd get the electron buildup you mentioned. Not only would such a buildup not be in the long-term interests of the viewer, the accumulated charge would deflect incoming electrons from their intended target, resulting in a highly distorted image. And quickly, too. The charge is bled off the inside of the jug through the connection illustrated.

-e

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#10
In reply to #2

Re: Where do the Electrons Go.....??

02/13/2007 5:41 AM

There is a transparent conductor (which doesn't have to have a low resistance because of the high voltage and low current involved). See http://en.wikipedia.org/wiki/Aquadag

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#3

Re: Where Do the Electrons Go?

02/12/2007 12:18 PM

A good question:

Those electrons that hit the phosphorous inside become light. Those electrons that do not hit anything become high frequency radiation, which is why there are warnings on those things saying that being two close can hurt you eyes.

You are setting on the receiving end of a gun spewing high frequency radiation just about all over the place. The inverse law says that the farther away the safer it should be, but who knows? possibly that is affecting all of our genes and the future will be filled with people that have arms growing out of their heads?

Kinda makes you want to get up and walk around once in while so your exposure is lessened.

See Ya :-)

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#4

Re: Where Do the Electrons Go?

02/12/2007 2:00 PM

i dont have advanced knowledge of quantic mechanic, so i gonna explain things as i know, and if im wrong please let me know hehe, when the electrons hit the screen the screen the screen emits light because is covered with a phosphorescent coating, in other words the electrons hit the screen and the screen irradiates fotons these fotons can be irradiated because they have absorbed great part of the electrons energy, other part is the static electricity of the scren, but where do the electrons go?, The interior side of the phosphor layer is often covered with a layer of aluminium. The phosphors is a poor electrical conductors, which leads to deposition of residual charge on the screen, effectively decreasing the energy of the impacting electrons due to electrostatic repulsion. The aluminium layer is connected to the conductive layer inside the tube, and disposes of this charge. Electrons dont make capacitors what make a capacitor is the the two conductors and the dielectric, as far as i know the capacitors are build by a conductive layer, a dielectric and other conductive layer so if the glass where the dielectric, where are the two conductive layer if one side is the atmosphere?

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#7

Re: Where Do the Electrons Go?

02/12/2007 11:31 PM

"Are they converted to photons?"

No, unless you're talking about electrons colliding with positrons (which you aren't), producing gamma rays and neutrinos. What they do basically is impart energy to the valence electrons in the phosphor molecules, forcing these electrons into higher orbits. After losing their kinetic energy in this way, what were the incoming electrons are then snatched-up by the positive voltage connected to the jug's internal metallic coating.

As Nature prefers the lowest-energy state possible, the phosphor's valence electrons drop back into their previously-held orbitals (their ground state) until such time as the phosphor is struck by another electron. But in order to return to their ground state, the valence electrons must somehow shed the energy imparted them by the fast-moving electrons, so they do so as light (and heat). The particular phosphor in question determines the wavelength that is emitted.

"...maybe they leak away through the glass to atmosphere?"

No, they're bled off as described earlier. The glass envelope of a CRT is much too good an insulator to permit the kinds of currents required to keep the CRT electrically neutral.

"...Could they just stay put and sit on the face of the tube until the tube is scrapped?"

No, ditto. Image distortion.

"We all know that touching the glass front of a TV tube gives a static 'tingle' as well as a crackle, could the electrons form a capacitor with the glass front of the tube as a dielectric?"

A residual charge remains on in the interior, metallic-film face of the CRT, forming one charged "electrode" of a capacitor. The other "electrode" consists of dirt, moisture - your hand - and so forth, on the other side of the screen, which "feels" the resulting electrostatic field through the glass.

-e

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#8

Re: Where Do the Electrons Go?

02/13/2007 12:15 AM

The explanation is relatively simple.

The cathode ray electron beam hitting the phosphorescent screen knocks secondary electrons out of the phosphor itself. Those are collected by the second anode coating within the CRT, and are responsible for second anode current.

Aluminization is a different matter. It is used to prevent ions from the cathode from degrading the phosphor. Early CRTs without aluminization had the electron gun at an angle to the tube axis, and used either a permanent magnet or electromagnetic "ion trap" on the neck of the CRT to deflect the ions off-screen. The electrons, of lower mass, were deflected to the on-screen area by the ion trap.

Bernie Katz

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#11
In reply to #8

Re: Where Do the Electrons Go?

02/13/2007 7:36 AM

Ahhhh Bernie, I had forgotten about the ion trap in the old tubes...

I've read through the other posts and was about to state that early tubes didn't have a pda and were plain glass on the inside and NOT metallised, so there was no leakage path across the glass to a side EHT terminal...

There still must be a build up of electrons on the phosphor to give such a high static charge to the front of the tube (as when your finger approaches the tube face there is a static charge build up). So maybe not sufficient electrons are displaced to keep the front phosphor screen free of a build up of electrons?

Incidentally, I started this discussion as for some reason CR4 had been getting a bit quiet recently so let's have an argument over this huh?!!!!!

John.

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#12
In reply to #11

Re: Where Do the Electrons Go?

02/13/2007 7:49 AM

I'm sorry! Is this the right room for an argument?

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#13
In reply to #12

Re: Where Do the Electrons Go?

02/13/2007 7:56 AM

You come in here and have to ask??

Sorry, is this a 5 minute argument or a 10 minute one?

John

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#15
In reply to #13

Re: Where Do the Electrons Go?

02/13/2007 8:01 AM

I thought it was the full half hour!

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#14
In reply to #12

Re: Where Do the Electrons Go?

02/13/2007 8:00 AM

"I'm sorry! Is this the right room for an argument?"

-----

This is Abuse. Silly git.

Getting Hit On The Head lessons are down the hall.

-e

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#16
In reply to #14

Re: Where Do the Electrons Go?

02/13/2007 8:02 AM

Abuse is what you shout out of the car window!

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#18
In reply to #16

Re: Where Do the Electrons Go?

02/13/2007 8:10 AM

Oh dear I thought I was the only one who was bored....

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#19
In reply to #18

Re: Where Do the Electrons Go?

02/13/2007 8:18 AM

How could I possibly be bored, working in a cutting edge envionmet as ...I...Do......Znnnnnn

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#20
In reply to #18

Re: Where Do the Electrons Go?

02/13/2007 8:22 AM

Not! But I must get ready for work and I've been procrastinating far too long.

Ciao, y'all!

-e

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#17
In reply to #12

Re: Where Do the Electrons Go?

02/13/2007 8:09 AM

M= Man looking for an argument (PlbMac),

R= Receptionist (English Rose),

Q= Abuser (Europium),

A= Arguer (Electroman),

C= Complainer (Shyam),

H= Head Hitter (Roger Pink)


M: Ah. I'd like to have an argument, please.
R: Certainly sir. Have you been here before?
M: No, I haven't, this is my first time.
R: I see. Well, do you want to have just one argument, or were you thinking of taking a course?
M: Well, what is the cost?
R: Well, It's one pound for a five minute argument, but only eight pounds for a course of ten.
M: Well, I think it would be best if I perhaps started off with just the one and then see how it goes.
R: Fine. Well, I'll see who's free at the moment.
Pause
R: Mr. DeBakey's free, but he's a little bit conciliatory.
Ahh yes, Try Mr. Barnard; room 12.
M: Thank you.

(Walks down the hall. Opens door.)

Q: WHAT DO YOU WANT?
M: Well, I was told outside that...
Q: Don't give me that, you snotty-faced heap of parrot droppings!
M: What?
Q: Shut your festering gob, you tit! Your type really makes me puke, you vacuous, coffee-nosed, malodorous pervert!!!
M: Look, I CAME HERE FOR AN ARGUMENT, I'm not going to just stand...!!
Q: OH, oh I'm sorry, but this is abuse.
M: Oh, I see, well, that explains it.
Q: Ah yes, you want room 12A, Just along the corridor.
M: Oh, Thank you very much. Sorry.
Q: Not at all.
M: Thank You.
(Under his breath) Stupid git!!

(Walk down the corridor)
M: (Knock)
A: Come in.
M: Ah, Is this the right room for an argument?
A: I told you once.
M: No you haven't.
A: Yes I have.
M: When?
A: Just now.
M: No you didn't.
A: Yes I did.
M: You didn't
A: I did!
M: You didn't!
A: I'm telling you I did!
M: You did not!!
A: Oh, I'm sorry, just one moment. Is this a five minute argument or the full half hour?
M: Oh, just the five minutes.
A: Ah, thank you. Anyway, I did.
M: You most certainly did not.
A: Look, let's get this thing clear; I quite definitely told you.
M: No you did not.
A: Yes I did.
M: No you didn't.
A: Yes I did.
M: No you didn't.
A: Yes I did.
M: No you didn't.
A: Yes I did.
M: You didn't.
A: Did.
M: Oh look, this isn't an argument.
A: Yes it is.
M: No it isn't. It's just contradiction.
A: No it isn't.
M: It is!
A: It is not.
M: Look, you just contradicted me.
A: I did not.
M: Oh you did!!
A: No, no, no.
M: You did just then.
A: Nonsense!
M: Oh, this is futile!
A: No it isn't.
M: I came here for a good argument.
A: No you didn't; no, you came here for an argument.
M: An argument isn't just contradiction.
A: It can be.
M: No it can't. An argument is a connected series of statements intended to establish a proposition.
A: No it isn't.
M: Yes it is! It's not just contradiction.
A: Look, if I argue with you, I must take up a contrary position.
M: Yes, but that's not just saying 'No it isn't.'
A: Yes it is!
M: No it isn't!

A: Yes it is!
M: Argument is an intellectual process. Contradiction is just the automatic gainsaying of any statement the other person makes.
(short pause)
A: No it isn't.
M: It is.
A: Not at all.
M: Now look.
A: (Rings bell) Good Morning.
M: What?
A: That's it. Good morning.
M: I was just getting interested.
A: Sorry, the five minutes is up.
M: That was never five minutes!
A: I'm afraid it was.
M: It wasn't.
Pause
A: I'm sorry, but I'm not allowed to argue anymore.
M: What?!
A: If you want me to go on arguing, you'll have to pay for another five minutes.
M: Yes, but that was never five minutes, just now. Oh come on!
A: (Hums)
M: Look, this is ridiculous.
A: I'm sorry, but I'm not allowed to argue unless you've paid!
M: Oh, all right.
(pays money)
A: Thank you.
short pause
M: Well?
A: Well what?
M: That wasn't really five minutes, just now.
A: I told you, I'm not allowed to argue unless you've paid.
M: I just paid!
A: No you didn't.
M: I DID!
A: No you didn't.
M: Look, I don't want to argue about that.
A: Well, you didn't pay.
M: Aha. If I didn't pay, why are you arguing? I Got you!
A: No you haven't.
M: Yes I have. If you're arguing, I must have paid.
A: Not necessarily. I could be arguing in my spare time.
M: Oh I've had enough of this.
A: No you haven't.
M: Oh Shut up.

(Walks down the stairs. Opens door.)

M: I want to complain.
C: You want to complain! Look at these shoes. I've only had them three weeks and the heels are worn right through.
M: No, I want to complain about...
C: If you complain nothing happens, you might as well not bother.
M: Oh!
C: Oh my back hurts, it's not a very fine day and I'm sick and tired of this office.


(Slams door. walks down corridor, opens next door.)

M: Hello, I want to... Ooooh!
H: No, no, no. Hold your head like this, then go Waaah. Try it again.
M: uuuwwhh!!
H: Better, Better, but Waah, Waah! Put your hand there.
M: No.
H: Now..
M: Waaaaah!!!
H: Good, Good! That's it.
M: Stop hitting me!!
H: What?
M: Stop hitting me!!
H: Stop hitting you?
M: Yes!
H: Why did you come in here then?
M: I wanted to complain.
H: Oh no, that's next door. It's being-hit-on-the-head lessons in here.
M: What a stupid concept.

-e

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Anonymous Poster
#21
In reply to #17

Re: Where Do the Electrons Go?

02/13/2007 10:02 AM

I expected an MP3 file of Peter, Paul & Mary singing "Where Have All The Electrons Gone?"

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Guru
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#22
In reply to #21

Re: Where Do the Electrons Go?

02/13/2007 10:20 AM
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#27
In reply to #17

Re: Where Do the Electrons Go?

02/15/2007 7:22 PM

Oh, THANK you!

This Pythonesque bit of argument is exactly what I needed. Better than coffee. It's been SUCH a boring day. (I've been proof-reading data sheets for memory chips, you can't imagine...)

Thanks, Europium. And remind me not to give you any reason for abuse!

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#28
In reply to #27

Re: Where Do the Electrons Go?

02/15/2007 7:52 PM

(sits down at computer and logs on)

A: This Pythonesque bit of argument is exactly what I needed.
E: No, it isn't.
A: Better than coffee.
E: It is not.
A: It's been SUCH a boring day.
E: No, it hasn't.
A: (I've been proof-reading data sheets for memory chips, you can't imagine...)
E: Yes I can. I proof-read and edited over 600 pages of a TI DSP manual for over three years one month.
A: YOU want to complain? Just look at all these data sheets...!!!

-e

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#29
In reply to #28

Re: Where Do the Electrons Go?

02/16/2007 2:32 AM

"Mr St.John Polevaulter, I understand you contradict people"
"NO, I DON'T!"
".. er.. but you told me that you did ..."
"I MOST CERTAINLY DID NOT!"
" Ohh.. I see. I'll start again ."
"NO YOU WON'T"
"I understand you don't contradict people"
"YES I DO!"
" . and when didn't you start? "
" 1952!"
" 1952 ?"
" 1947!!"
" 60 years ago, then ?"
"NO !!"

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#30
In reply to #29

Re: Where Do the Electrons Go?

02/16/2007 4:12 AM

I agree.

-e

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#31
In reply to #29

Re: Where Do the Electrons Go?

02/16/2007 4:13 AM

No I don't.

-e

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#32
In reply to #29

Re: Where Do the Electrons Go?

02/16/2007 4:14 AM

Yes I..no I..

Oh dammit all!

Where was I?

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#33
In reply to #32

Re: Where Do the Electrons Go?

02/16/2007 5:10 AM

I'm not indecisive! Or am I?

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#34
In reply to #33

Re: Where Do the Electrons Go?

02/16/2007 5:23 AM

Here's what to do: snuggle up with a tattered copy of "Ethyl The Aardvark Goes Quantity Surveying," by Charles Dikkens.

No worries - not even about the baggage-retrieval system they've got at Heathrow.

-e

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#35
In reply to #34

Re: Where Do the Electrons Go?

02/16/2007 5:27 AM

No worries - not even about the baggage-retrieval system they've got at Heathrow.

Now your getting into advanced quantum mechenics and imaginary higher math. Over to Jorrie, I think.

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#36
In reply to #35

Re: Where Do the Electrons Go?

02/16/2007 5:33 AM

Jorrie?! Oh, pleeeeze! Not Jorrie!!!

Quick! Get me the Comfy Chair!

-e

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#23
In reply to #11

Re: Where Do the Electrons Go?

02/13/2007 12:02 PM

Google :Aquadag coating.

The answer is here if you really want to know.

HTRN

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#26
In reply to #11

Re: Where Do the Electrons Go?

02/14/2007 2:41 AM

Electroman:

Yes, it is easy to forget about older aspects of technology, and all else for that matter. But I "cut my electronic teeth" so to speak, as an early teenager who experimented with an old RCA model 630 chassis (1947 era) night after night. So it is very easy to remember the ion trap, focus coil, and the shape of the deflection coil windings on the neck of the 10BP4 CRT!

You are correct in stating that some of the early CRTs were not internally metallized (the coating was actually formed from an aqueous carbon suspension) for a second anode connection. While more popular in lower cost oscilloscopes, so-called monoaccelerator CRTs did not use a second anode coating for post-deflection acceleration. Many Telequipment scopes used monoaccelerator CRTs, as did some of the lower cost Tektronix scopes (like the 561) in the USA.

One television receiver I had as a kid was an electrostatic deflection 7" set made by Motorola in the late 1940's or very early 1950's. It used a 7AP4 CRT if I'm not mistaken, also a monoaccelerator. However, I do not recall ever seeing a magnetically deflected CRT without post-deflection acceleration.

Let's have a toast to the old days of simplicity.

Bernie Katz

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#24

Re: Where Do the Electrons Go?

02/13/2007 8:39 PM

The CRT phosfhor has an aluminum layer behind it to reflect light that would have been going back into the tube, to increase the brightness. Don't ask me how the electrons get thru it. It wasn't always so - CRTs did not always have the aluminum, trust an old fart on this. Of course electrons get converted to photons, or you wouldn't be able to SEE anything, duh!

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#25
In reply to #24

Re: Where Do the Electrons Go?

02/14/2007 1:37 AM

"Of course electrons get converted to photons, or you wouldn't be able to SEE anything, duh!"

-----

From one Old Fart to another Old Fart (betcha I'm older). There's more to this question than meets the..um..eye?

See my previous post (and others) concerning this conversion...

Ciao, Young Fart!

-e

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#37

Re: Where Do the Electrons Go?

05/01/2014 11:31 AM

I'm going to revive this old topic, just to add some references.

@berniek: consider upgrading from the 10BP4 to the 10SP4.

Where do the electrons go? Some go to the first anode, maybe as much as 1-5mA, and to the remaining anodes at lower currents, but if referring to the screen, and what we call the PDA or Ultor coating, the current is much lower there. It depends on the tube but 1mA or less is reasonable, and in scope CRTs, the range is usually 50-100uA. Some tubes have a dissipation rating implied for the screen. Maybe 4-6 Watts max. IIRC from the CRT sections of older tube manuals.

Looking at scope CRTs and even the TV set types, when the aluminum coating is added to the tube to prevent ion burns and increase brightness, the high voltage must be increased to overcome (penetrate) it. brightness and HV level can be increased even more because the aluminized screen won't burn easily, and the result is many times the original brightness compared to the non-aluminized CRT. This is true in Older vs. newer monochrome TV sets and in oscilloscopes. This is why a Tektronix 545 CRT runs 10KV and is blindingly bright, and an old DuMont 304A scope runs 3KV and is hard to see across a well-lit room. The aluminum also acts as a heat sink. 1-3KV more is required, depending on the thickness of the aluminum coating.

Five great books on the topic of CRTs are these:

"Cathode Ray Tube Displays" - MIT Radiation Lab Series, Volume 22 (download online or buy the real thing on amazon/ebay). This book covers the use of CRTs in displays, including deflection coils, sweep circuits and other considerations for very diverse applications of X-Y display use. It can be found on electronicsandbooks.com. Below is the link to the book, but I suggest looking over the whole site for interesting reads. http://www.electronicsandbooks.com/eab1/manual/Publisher/M/MIT/Cathode%20Ray%20Tube%20Displays,%20T%20Soller%201948%20MIT%20Radiation%20Laboratory%20Series%2022%20%5B764%5D.pdf . It can also be found along with all of the other MIT volumes of the series, (copyrights are expired) on this hobby site, which I also recommend having a look at, bunkerofdoom.com, "Where doomed technologies hide from the present". The PDF is here: http://bunkerofdoom.com/lit/mitser/V22.PDF and it is also on archive.org as https://archive.org/details/CathodeRayTubeDisplays .

"The Cathode Ray Tube" by Peter Keller. Palisades press. This book is covers the evolution of the CRT and describes its many refinements and subtypes. This is my second favorite CRT book. Mr. Keller worked for Tektronix. This book should be bought directly from Mr. Keller according to http://www.crtsite.com/page2.html , and one can order it via e-mail: kellerp4(at)frontier(dot)com. I provide this info because amazon sellers are confused and want US800+ for it.

"Cathode Ray Tubes" by Manfred Von Ardenne (original in German, also translated into English and published in the UK). Covers the earlier evolution of the CRT with greater detail on the experiments at the very beginning. This book is from 1939, but the copy I have was done on good paper so it's going to last.

"Cathode Ray Tubes" by Tektronix. 1968. One of the Circuit Concepts series. A smaller size book that covers the way that high performance CRTs are operated and what the supporting circuits do. It specifically charts the engineering concepts and improvements made to Tek oscilloscope CRTs which are of course reflected throughout the industry.

"Servicing TV Horizontal and Vertical Output Systems" focuses more on TV sets. This one explains in great detail how the deflection systems work including features like linearity, tuning, and other functions necessary to generate the best quality raster. I think this one is also on line or avail. cheap via amazon.

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